1. Field of the Invention
The present invention relates to a method for continuous casting of steel, and more particularly to a method for controlling a flow of molten steel fed from an immersion nozzle into a mold for continuous casting of steel by the use of magnetic force.
2. Description of the Prior Arts
FIG. 7 is a schematic illustration showing a flow of molten steel from an immersion nozzle into a mold in a slab continuous caster. Mold powder floats on the surface of the molten steel 8 inside the mold 1. The mold powder presents the molten steel 8 from being oxidized, provides thermal insulation of the molten steel 8, provides lubrication between solidified shell 9 and the mold 1 and absorbs non-metallic particles in the molten steel. The mold powder on the side of molten steel surface is in the state of being melted by the heat of the molten steel 8. The mold powder on the atmospheric side covers the surface of the molten steel 8 in the form of powder 7. Molten powder 6 flows between the solidified shell 9 and the mold 1 and plays the role of a lubricant. The molten powder 6 is replenished at a rate of its consumtion since it is consumed as the libricant. The thickness of the mold powder layer is controlled to be a predetermined value. Immersion nozzle 2 is vertically positioned at the central portion of the mold 1. Exit ports 3 arranged at the end of the immersion nozzle 2 have an opening facing narrow side walls of the mold 1. The molten steel is poured from the exit port 3. Flow 4 of the poured molten steel moves downward obliquely toward the narrow side wall of the mold. The flow 4 of the poured molten steel strikes the narrow side wall of the mold and is divided into an upward flow and a downward flow, that is, turn-over flow 11 and penetration flow 12. The turn-over flow 11 rises along the narrow side wall of the mold and becomes a cause of a wavy motion of a molten steel surface near the narrow side wall of the mold. FIG. 8 is a schematic illustration showing the wavy motion of molten steel surface inside the mold. The flow poured from the exit port 3 of the immersion nozzle 2 is divided into the turn-over flow 11 and the penetration flow 12. The turn-over flow 11 reaches the molten steel surface and causes the level of the molten steel surface to fluctuate. Fluctuation of the molten steel surface gives rise to the wavy motion of the molten steel surface. The wavy motion of the molten steel surface is measured by means of eddy current type distance measuring device 15. The voltage signal is filtered, by which high frequency elements are removed. The voltage signal, from which the high frequency elements have been removed, is measured by means of a millivoltmeter. The eddy current type distance measuring device 15 is arranged above the molten steel surface near the narrow side of the mold as shown in FIG. 8. FIG. 8 is a schematic illustration showing the wavy motion of the molten steel for about one minute. The molten steel surface continuously rises or falls. The level of the wavy motion of the molten steel for one minute is measured. The maximum value of the level of the wavy motion of the molten steel is regarded as the maximum height "h" of a wave of the molten steel surface and a data processing is carried out. In a high rate casting, wherein molten steel of 3 ton/min or more is poured, a flow rate of molten steel poured from the exit port 3 of the immersion nozzle 2 is large. The turn-over flow 11 of molten steel which is produced after the flow of poured molten steel has struck the solidified shell 9 also is large and causes a large wavy motion of molten steel to be formed. FIG. 10 is a graphical representation designating the relationship between the maximum height of the wavy motion of molten steel surface and the index of surface defect of hot-rolled steel plate. As clearly seen from FIG. 10, the ratio of occurrence of the surface defect of hot-rolled steel plate is small when the maximum height of wavy motion of molten steel surface is within a range of 4 to 8 mm. The range of 4 to 8 mm of the maximum height of wavy motion of molten steel surface is preferable. In case the wavy motion of molten steel surface is large, molten powder 6 is easily trapped by the molten steel by the wavy motion of molten steel surface and suspended in the molten steel. The molten powder 6 having been trapped by the molten steel rises on the surface of molten steel due to a difference in the specific weights of the molten steel and the molten powder 6, but some of the molten powder 6 is caught by the solidified shell 9. On the other hand, when the wavy motion of molten steel surface is small, a small amount of new molten steel is fed to the molten steel surface. In consequence, the mold powder 5 is hard to melt. Accordingly, it is hard for the inclusions to be melted and adsorbed into the molten powder 6. The inclusions are caught by the solidified shell 9 and are liable to be inner defect of a slab. The values of 4 to 8 mm which are the preferable range of the maximum height of molten steel surface were obtained by experience in operations of continuous casting. The form and the pouring angle of the immersion nozzle 2, clogging in the immersion nozzle 2 and the width of the mold 1 are specified so that the maximum height of wavy motion of molten steel surface can be within said range.
Recently, however, the operations shown below have been carried out and operation conditions have changed to increase productivity in the continuous casting of steel.
(a) The multiple continuous casting of steel in which several charges of casting are continuously carried out by the use of one tundish and one immersion nozzle. PA1 (b) The change of widths of mold during the continuous casting of steel. PA1 (c) The change of casting rate from a low value to a high value.
As the result of the change of the aforementioned operation conditions, the form and the pouring angle of the immersion nozzle, set for the initial operation, does not fit to the successive operation conditions, which leads to the incapability of the control of the level of the molten steel to the most pertinent range.
Two methods are known to control the height of wavy motion of a molten steel surface. The prior art method 1 disclosed in Nagai Iron and Steel 685270(1982) is a method wherein a flow of molten steel poured from two exit ports is braked by a direct current magnetic field. Two pairs of direct current magnets are arranged inside a cooling box positioned on a surface on the wide side of a mold and introduce a direct current magnetic field to the flow of molten steel poured from the immersion nozzle. The flow of molten steel is controlled by magnetic force produced in the direction opposite to the flow of molten steel induced in the molten steel by the electric current and direct current magnetic field. The prior art method 2 is a method wherein direct current magnetic field is introduced at the molten steel surface. The height of wavy motion of the molten steel surface in the magnetic field is controlled by arranging a direct current magnet at the position of the molten steel surface and horizontally introducing the direct current magnetic field to the molten steel surface. The prior art method 1 is disclosed in "Iron and Steel" (1982), Nagai et al., 68, S 270, and "Iron and Steel" (1982), Suzuki et al., 68, S 920. The prior art method 2 is disclosed in "Iron and Steel" (1986), Ozuka et al., 72, S 718.
The flow of molten steel poured from the immersion nozzle strikes the solidified shell and is divided into an upward turn-over flow and a downward penetration flow. Since kinetic energy which the upward turn-over flow has oscillates the molten steel surface, a wavy motion of the molten steel surface is formed.
However, in the prior art method 1, a direct current magnetic field is introduced perpendicular to the flow of molten metal poured from the immersion nozzle only in the portion between the immersion nozzle and the surface of the narrow side of the mold. The flow of molten metal is braked. In this method, because the flow disperses after it has been poured from the immersion nozzle, and thus a strong direct current magnetic field has to be introduced to control the dispersing flow of poured molten steel. Since the direct current magnetic field is required to control the wide range of dispersing flows in the poured molten steel, large sized equipment is required, by which the production cost is increased. Moreover, in prior art method since a circuit of the eddy current, formed by the mutual work of the flow of molten steel with the direct current magnetic field, is formed in the molten steel in this method, the current density cannot be increased. Accordingly, to generate a great braking force, the magnetic flux density should be increased. The cost of the equipment is increased to increase the magnetic flux density.
The wavy motion is most easily controlled in the prior art method 2 since the direct current magnetic field is directly introduced against the wavy motion of molten steel surface. However, the position where the wavy motion of the molten steel surface is most violent is situated within the range of 100 mm from the narrow side of the mold. Accordingly, the direct current magnetic field is introduced to the range of 100 mm from the narrow side of the mold. A device for generating a magnetic field is required to be placed on the reverse side of a wide side copper plate of the mold and in the position about 100 mm away from the upper end of the wide side of the mold. In case when the device for generating a magnetic field is placed in the above-mentioned position, a large scale revamp of the cooling box is necessary and the direction of cooling path on the copper plate of the mold is required to be made transverse. Then, the wide side copper plate of the mold is insufficiently cooled.